Tachyarrhythmias are episodes of high-rate cardiac depolarizations. Tachyarrhythmias may occur in one chamber of the heart or may be propagated from one chamber to another. Some tachyarrhythmias are sufficiently high in rate to compromise cardiac output from the chamber affected, leading to loss of consciousness or death in the case of ventricular fibrillation, or weakness and dizziness in the case of atrial fibrillation. Atrial fibrillation is often debilitating, due to the loss of atrial cardiac output, and may sometimes lead to ventricular fibrillation.
Fibrillation may be terminated by administering high energy level cardioversion or defibrillation shocks until the fibrillation is terminated. For example, an implanted device may deliver defibrillation shocks via an electrode carried by a lead implanted within the heart. Unfortunately, the high energy levels associated with cardioversion/defibrillation shocks can cause significant pain to the patient. In addition, atrial defibrillation shocks can sometimes give rise to ventricular arrhythmias. Therefore, it is generally desirable to avoid the onset of atrial fibrillation, and the need to apply defibrillation shocks.
Some implanted devices deliver anti-tachycardia pacing pulses to terminate detected episodes of atrial tachycardia. Other devices are configured to predict the onset of atrial fibrillation, and deliver pacing pulses to prevent the atrial fibrillation from occurring. In particular, a device may be configured to detect premature atrial contractions (PACs) as trigger events that indicate the onset of atrial fibrillation. Delivery of pacing pulses in response to PAC detection can help prevent or decrease the occurrence of atrial fibrillation. Pacing pulses delivered in response to PAC detection are sometimes referred to as post-PAC pacing pulses.